1. Field of the Invention
This invention relates to methods of fracturing hydrocarbon containing subterranean formations, and particularly to methods of controlling scaling in such fracturing operations.
2. State of Art
One common cause of production declining in a mature hydrocarbon well is fouling of the perforations in the well casing and the structure of the formation around the well with scale precipitated from brine frequently found down hole. The production may be revived, at least partially, with a stimulation technique. One commonly used technique is hydraulic fracturing. In the process of hydraulic fracturing, a fracturing fluid is injected at high pressure into a subterranean formation to create artificial cracks in the subterranean formation. A propant added to the fracturing fluid fills the fractures to maintain the openings created by the crack. Hydraulic fracturing not only exposes new rock, but it also tends to break scale down hole. Although the fracture exposes new rock and breaks scale, once the fracture has been made and hydrocarbon production resumed, the well and the adjacent subterranean formation are still subject to scaling from precipitating minerals from subterranean brines, for example, calcium sulfate and calcium carbonate.
Since hydraulic fracturing is costly, sometimes costing as much as drilling the well in the first place, it is necessary that future build-up of scale be prevented as much as possible. One common solution is performing two separate operations, a fracturing operation followed by a scale inhibiting operation. Using a second operation to inhibit scale in the subterranean formation after the fracturing operation, does not provide an effective method for preventing scale in most wells. In these wells, the subterranean formation, as well as the perforations in the well casing, are plugged by scale. However, the two-step process contacts only that portion of the hole nearest the well with scale inhibitor. Because such a small portion of scale inhibitor is injected, most of the fracture is not contacted by scale inhibitor. Furthermore, the amount of scale inhibitor injected into the hole may be rapidly depleted. Then the well operator is faced with a second descaling operation, or even a second fracture job. In "Removal and Inhibition of Calcium Sulfate Scale in Water Flood Projects," C. F. Smith et al., J. Petroleum Technology, Nov. 1968, p. 1219 the general solution of a two-step descaling operation using a phosphonate scale inhibitor to prevent calcium sulfate scale is discussed.
A fracturing fluid that prevents or inhibits scale formation would be extremely useful in the oil field. Such a fracturing fluid would put the scale inhibitor deep into the fracture, in a one-step operation. But such a fracturing fluid must, on the one hand, preserve the viscosity of cross-linking gel, and on the other, preserve the activity of the scale inhibitor. And, a significant amount of scale inhibitor should adsorb to the subterranean formation rock.